The present invention relates to a method and system for aligning optical fibers to a lens array, and relates particularly to, a method and system for aligning optical fibers to a lens array in which each fiber is aligned at the focal point of a different lens of the array and then attached to the array, such that beams emitted from lenses of the array are pointing in the same direction, i.e., parallel to each other, when light is provided to their respective fibers. The invention further relates to a collimator array provided by the assembly of the lens array and such aligned optical fibers.
In the fabrication of optical devices, it is often desirable to produce a set of collimated optical signals (e.g., beams), each encoded with information from a different source. Monolithic one or two-dimensional arrays of collimating elements may be used in which each collimating element is coupled to a different optical path (e.g. an optical fiber) to provide this set of collimated optical signals. Such a lens array, referred to as a microlens array herein, may consist of a plurality of lens elements formed into a single substrate, or plate, of material, or integrated onto such substrate. This material may be, for example, of plastic (polymers), glass, silicon, or silica. Each optical fiber is attached to the back surface of the substrate for illuminating a different one of the lenses in the array. The end of each optical fiber should be aligned at the focal point of its respective lens to enable optimal light beam collimation, focusing, or maximum light coupling (minimum insertion loss) into another similarly produced lens. Moreover, such alignment should provide collimated beams aligned parallel to each other from lenses when light is provided to their respective fibers. Positioning the optical fibers so that the optical beams emerging from the collimator array are highly collimated and collimated parallel to each other is a difficult task, becoming even more difficult with increased density of lens in the array.
One possible method of alignment is to use a complex structure of a matrix of optical fibers which may be provided by threading optical fibers into holes of a substrate and then polish the ends of the fibers to be aligned to the array. The complex structure is oriented and attached to the lenses of the array to couple illumination from fibers to lenses of the array. This however does not assure that the ends of the fibers are each properly located at the focal point of each lens, or that one of more of the ends of the fibers are not tilted to effect orientation of beams from lenses.
The complex structure for fiber alignment may be fabricated into one side of the substrate containing the microlens array. For example, U.S. Pat. No. 5,346,583 describes a method for aligning optical fibers with a microlens array in which one side of a substrate contains the microlens array and the other side has an array of circular apertures each aligned with the central axis of one of the lenses of the array. Optical fibers are inserted into these apertures, such that the ends of the fibers are aligned in a common plane with respect to the lenses. The substrates"" sides are produced by a photolithographic mask and etching processes on each side of the substrate. This method thus requires two masks, which must be precisely aligned with each other, otherwise the central axis of the lenses will not align with the circular apertures. Laser beams are directed through a slot in each mask for mask-to-mask alignment. This may improve enmasse alignment of optical fibers, but such manufacture increases the cost of the microlens array, and does not account for variations which often occur in the focal length between different microlenses of the array. Thus, it would be desirable to align multiple fibers to a microlens array, without requiring a complex structure for enmasse fiber alignment.
In fiber optic connections, techniques have been developed for aligning an individual optical fiber to a GRIN (graduated refractive index) lens. For example, in U.S. Pat. Nos. 4,509,827 and 4,545,643, a mirrored surface is positioned substantially orthogonal relative to the axis of a GRIN lens to autocollimate a light beam transmitted through a fiber. The fiber is positioned relative to the lens such that the returned signals from the mirrored surface is maximized. An adhesive then secures the fiber to the lens. In a further example of fiber optics connection, U.S. Pat. No. 4,637,683 aligns an optical fiber to a GRIN lens having a reflective surface coating on the side of the lens opposite from the fiber, where the reflective surface provides a reference plane. The optical fiber is positioned relative to the GRIN lens to maximize the reflected light from the reference plane. Such methods of U.S. Pat. Nos. 4,509,827, 4,545,643 and 4,637,683 are limited to alignment of a single fiber to a single GRIN lens rather than alignment of multiple optical fibers to a microlens array.
Other alignment methods for aligning an individual fiber to a lens use transmitted, rather than reflected light. U.S. Pat. No. 5,009,482, describes joining an optical fiber with a spherical lens by detecting the amount of light transmitted by the lens into the fiber, and iteratively positioning the fiber relative to the lens to maximize the amount of detected light. In European Pat. Publication EP 0619505B1, multiple separate GRIN lens are aligned with optical fibers in a structure mechanically providing an optical collimator array. Light is passed along the optical fibers and beams emitted from the GRIN lenses are imaged on a CCD camera and shown on a monitor coupled to the CCD camera. The centers of the beams in the image are used to adjust the position of fibers and lens to provide the desired output from the optical collimation array.
Often optical fiber have a ferrule coupled about one of their ends having a front surface planar with the end of the fiber to be attached to a microlens array. In attaching individual fibers to the array, excess adhesive used in joining the fiber to the array protrudes from the location where the ferrule attaches to the back surface of the lens array""s substrate. Often such protruding adhesive forms a bead or runs along the back surface of the array""s substrate. This can be a problem since the protruding adhesive can interfere with attachment of other neighboring fibers to the array. Thus, it would be desirable to avoid protruding adhesive in the attachment of fibers to the microlens array.
Accordingly, it is the principal object of the present invention to provide an improved method and system for aligning optical fibers to a lens array in which each fiber is aligned to a different lens to obtain proper collimation, focusing, or maximum light coupling, without enmasse alignment techniques of the prior art.
It is another object of the present invention to provide an improved method and system for aligning optical fibers to a lens array to obtain optical signals or beams aligned parallel to each other from lenses of the array when respective fibers receive illumination providing such optical signals.
A further object of the present invention is to provide an improved method and system for aligning optical fibers to a lens array in which alignment can be performed either manually, or automatically by a programmed computer.
A still further object of the present invention is to provide an improved method and system for aligning optical fibers to a lens array which avoid excessive adhesive used in joining fibers to the lens array from protruding upon the back surface of the lens array and interfering with attachment of other neighboring fibers to the array.
Briefly described, the present invention embodies a method for aligning optical fibers to a lens array in which the lens array has a substrate with a front surface providing a plurality of lenses in the array, and a back surface for input (or output) of light for the lenses. The method includes providing a planar reflective surface facing the front surface of the lens array, aligning the planar reflective surface substantially parallel with the front surface of the lens array such that the optical axes of the lenses of the array are substantially perpendicular to the planar reflective surface, locating the end of one of the fibers to be aligned adjacent the back surface of the lens array to face one of the lenses of the array, applying an adhesive material, such as ultraviolet light curable liquid adhesive, to the end the fiber, propagating light through the fiber and the lens facing the fiber to the reflective surface, and receiving returned reflected light from the reflective surface through the one of the fibers and the lens facing the fiber, adjusting the position the end of the one of the fibers to change the amount of the returned reflected light received by the fiber to determine when the end of the fiber is at a position which provides a maximum (or peak) amount (or power) of the returned reflected light, and attaching the end of the one of the fibers to the back surface of lens array at the position which provides a maximum amount (or power) of the returned reflected light. Such attachment may be facilitated by using a radiation source that provides ultraviolet light to cure liquid adhesive, in the case where an ultraviolet light curable adhesive is used. The propagating, adjusting, and attaching steps are repeated for each of the fibers to different ones of the lenses of the array until all fibers are coupled to the lenses of the array. Thus, a single reflective surface is provided and each fiber is aligned to maximize the reflected light from this reflective surface received through the fiber and its respective lens, and then the fiber is attached to the substrate. Since each of the aligned fibers and lenses are aligned to the same reflective surface, their beams will be parallel to each other when their respective fibers are illuminated. Although preferably the planar reflective surface is substantially parallel with the substrate of the lens array, the reflective surface need only be in a fixed relationship with the lens array during alignment and attachment of the fibers to the substrate to assure that such beams will be parallel with respect to each other.
A ferrule, or other type of coupler or connector, is provided about each end of the fibers when aligned and attached to the substrate of the lens array. Each ferrule may have one or more regions (structures or features) for retaining excessive adhesive joining the fiber to the lens array to avoid such excessive adhesive protruding upon the back surface of the substrate and interfering with placement of other fibers to the lens array. These regions are located at the front portion or surface of the ferrule facing the array""s substrate and provided by a chamfered front surface, a groove providing an annular canal in the front surface of the ferrule, or a combination thereof. The front surface of each ferrule may be angled with respect to the back surface of the substrate to provide the regions with or without being chamfered or having a canal. Alternatively, the fibers may be attached to the lens array without ferrules.
A system embodying the method for aligning optical fibers to a lens (or microlens) array is also provided, including, a lens array having a substrate with a back surface and a front surface providing the lenses of the array, optical fibers each having two ends in which one of ends may be positioned in a ferrule to provide a ferruled fiber, a reference member having a planar reflective surface facing the front surface of the lens array in which the front surface of the lens array is parallel to the reflective surface of the reference member such that the optical axes of the lenses of the array are substantially perpendicular to the planar reflective surface. A vacuum actuated holder is provided capable of retaining the fiber in its ferrule. The holder is pivotable such that when a fiber in its ferrule is retained, the ferrule faces the substrate in a fixed relationship, preferably parallel, to the substrate. Each of the fibers are individually loaded onto the holder and positioned to face a different one of the lenses of the substrate, in which the holder has translation stages capable of moving the end of the fiber in x,y,z orthogonal dimensions. At the end of the fiber an adhesive material is applied, such as an ultraviolet light curable adhesive, with a precise syringe. Each fiber when located adjacent the substrate, a laser beam is directed (propagated) through the fiber and the lens facing the fiber to the reflective surface, and returned reflected light from the reflective surface of the reference member passes through the fiber and the lens facing the fiber onto a detector. Optics are provided to pass light to each fiber and direct returned reflected light to a detector for measuring the amount (or power) of the reflected light. The stages coupled to the holder are each iteratively moved in x, y, or z to adjust the position of the end of the fiber to change the amount (or power) of returned reflected light received by the fiber to determine when the fiber in the coupler has moved to a position which provides a maximum amount (or power) of returned reflected light. The fiber is bonded to the back surface of the substrate at the position which provides a maximum amount (or power) of returned reflected light, such as applying ultraviolet light when an ultraviolet curable adhesive is used. The fiber is released from holder and another fiber is positioned in the holder, and then aligned and bonded to the array""s substrate, and so forth until all fibers are attached to the array. For each fiber to be aligned, the lens array is indexed to the next lens in the array by adjusting the position of the substrate of the lens array by using x,y translation stages coupled to the substrate, or the holder of the fiber may be moved using its x,y stages to index to the next lens.
An autocollimator, interferometer, or other aligning mechanism, may be used to locate a reference member providing the planar reflective surface substantially parallel with respect to one or more flat reflective areas on the front or back surface of the lens array""s substrate. The autocollimator or interferometer may be used during the alignment of each fiber to assure that the reflective surface is maintained substantially parallel with the substrate.
The movement of the stages of the holder to locate the position of maximum light reflectance may be carried out manually, or automatically by a computer system coupled to the stages and programmed to locate the maximum level (or power) of reflected light. The computer system operates the stages of the holder as a robotic arm to pick up each fiber by its ferrule from a fixture or cassette and locate and align the fiber when in the holder to one of the lenses in the array, as described above. The computer system controls vacuum to holder to retain the fiber in the holder and release the fiber after attachment to the lens array. The computer system may further control stages coupled to the precision syringe, such that prior to alignment of each fiber facing the lens array, the tip of the syringe may be positioned and adhesive applied to the fiber. After alignment of each fiber to a lens array, the computer system may also control the light source for curing the adhesive to bond the fiber to the substrate.
In addition to providing proper alignment, the system may be used to enable measurement of insertion loss for each fiber-lens pair as the lens array is assembled with fibers. This may be achieved by recording the power of the reflected light after each fiber is aligned and attached to the substrate.
A collimator array is also provided having such aligned optical fibers including, an array of lenses located on a substrate having a substantially flat back surface, and optical fibers each having one end coupled by an adhesive material to the back surface of the substrate in optical communication to a different one of the lenses in which each of the fiber are individually positioned with respect to the array. Each of the fibers may have a ferrule having regions for retaining excess adhesive material joining the fiber to the lens array to avoid the adhesive protruding upon the back surface of the substrate.
Although the lens array is described as having convex lens or elements for collimating light from fibers, and are not limited to collimating lenses, as such lens arrays may have lens for converging (or focusing) light. Further, the lens may operate to receive optical signals into the fibers or send optical signals received from the fibers. The lenses of the array may be surface relief, gradient index, or GRIN type lenses, or a combination thereof.